# Newton's Second Law-Pulley problem

1. Oct 11, 2007

### Precursor

1. The problem statement, all variables and given/known data
A 4.0-kg oak block on a horizontal, rough oak surface is attached by a light string that passes over a light, frictionless pulley to a hanging 2.0-kg object. The magnitude of the force of friction on the 4.0-kg block is 11.8 N.

2. Relevant equations
$$F_{net}$$=ma
$$F_{g}$$=mg

3. The attempt at a solution
I used $$F_{net}$$=$$F_{g}$$+$$F_{F}$$ and substituted in the values, but I don't seem to get the correct answer.

2. Oct 11, 2007

Either I am blind...or I don't see a QUESTION...

3. Oct 11, 2007

### dvdqnoc

What are you looking for? Tension?

4. Oct 11, 2007

### dvdqnoc

Assuming you're looking for tension...

5. Oct 11, 2007

### Precursor

I was looking for acceleration.

6. Oct 11, 2007

### dvdqnoc

Ok. Well, do you see how you can use my demonstration to solve for acceleration as well? Put T on one side of both italicized equations, set them equal to each other, then solve for acceleration.

7. Oct 12, 2007

### temaire

I am in the same class as Precursor, and I am also stuck on this question. I was able to get the acceleration, but I am unable to get the tension in the string. I got 15.7N. Dvd, you also got 15.7N, but our answer key for this assignment says that the answer is 17N. So, are we wrong or is our answer key wrong?

8. Oct 12, 2007

What did you calculate for an acceleration?

Casey

9. Oct 12, 2007

### temaire

I calculated $$1.3 m/s^{2}$$ for the acceleration.

10. Oct 12, 2007

### dvdqnoc

Perhaps your Force of Friction is incorrect? Double check and make sure all the numbers given is the same, and get back to me.

11. Oct 12, 2007

### chocokat

Dvdqnoc - I apologize in advance in case I've missed something, but, on the right side of your calculations, you have

T - 11.8 = 2a

Should it be

T - 11.8 = 4a

since that block has a mass of 4g?

12. Nov 20, 2008

### lightsyrup

Thank you - this was kind of helpful :)